Various position encoders for sensing linear, rotary or angular movement are currently available. These encoders are generally based on inductive, capacitive, optical, or magnetic transducers. In general, such encoders typically comprise a transducer comprising a readhead and a scale that includes a periodic structure having a characteristic spatial wavelength. The readhead may comprise a transducer element and some transducer electronics. The transducer output signals vary as a function of the position of the readhead relative to the scale along a measuring axis. The transducer electronics may output raw position signals to a signal processor or, process the raw signals internally and output modified signals indicative of the position of the readhead relative to the scale. It is also common for an encoder system to include an interface electronics separate from the readhead, and to interpolate or otherwise processes the transducer signals in the interface electronics before outputting modified signals indicative of the position of the readhead relative to the scale to an external host system, such as a motion control system or other data acquisition system.
Many conventional position encoder systems output either raw or modified position signal information in the form of two quadrature signals. As an example, a two-phase transducer may provide two raw position signals S1 and S2 that vary sinusoidally as a function of the position of the readhead relative to the scale along the measuring axis. In general, the signals S1 and S2 are intended to be identical except for a quarter-wavelength (quadrature) spatial phase difference between them. The relationship between the instantaneous values of the quadrature signals may be used to derive the instantaneous position of the readhead relative to the scale along the measuring axis, within a current period of the scale's spatial wavelength. In many encoders, such sinusoidal signals are passed through a threshold circuit to provide digital quadrature signals, that is, periodic square wave signals having a 90° spatial phase difference. In many other encoders and/or interface circuits the ratio between such sinusoidal signals, which behaves as a tangent function that depends on the position of the readhead along the scale, is analyzed to determine an “interpolated” position with a high resolution that is much finer than the scale wavelength (e.g. sub-micron resolution). However, because many motion control systems are designed to receive position information in the form of digital quadrature signals, it remains conventional to output such interpolated position information in the form of periodic digital quadrature signals. Interferometer systems are another type of device that may output very high resolution quadrature signals.
Regardless of the device that generates the quadrature signals, in order to keep track of such periodic quadrature signals and determine an accumulated total displacement value at any point in time, host systems such as displacement measuring systems, motion control systems, and the like, typically use a so-called quadrature counters. A quadrature counter, which includes a tracking counter, is utilized to decode the quadrature signals and to keep track of the changing position of the encoder. U.S. Pat. No. 4,599,600 to McGuire, et al., and U.S. Pat. No. 4,628,298 to Hafle et al., each of which is incorporated herein by reference, describe various aspects of quadrature decoding and related counters.
However, the trend in position encoders and other displacement measuring devices is to support increasing position resolution and higher motion speed. This makes the tracking counter a critical component of a position or displacement measuring system, in that its size (number of bits) and speed can limit the overall system capability. In other words, in order for the system to have a desirable level of position resolution while following relatively high speed motion, the tracking counter may need to be large and may be required to have a relatively high speed. It is also desirable for the tracking counter to have a relatively low cost so that the overall position encoder is economical. Conventional design techniques for such counters tend to produce a counting frequency which is too slow. As a specific example, a present FPGA counter with a desired cost-point (e.g. $15) with a desired resolution (e.g. 32 bit), may have a limited counting frequency (e.g. 20-40 MHz), whereas a desirable counting frequency would be higher (e.g. 100 MHz or more).
The present invention is directed to a quadrature counter that overcomes the foregoing and other disadvantages. More specifically, the present invention is directed to a high speed quadrature counter that is economical and is able to produce a desired counting frequency.